When aluminum alloy is melted to be cast as an aluminum alloy product, pores are often formed within a casting due to hydrogen gas or shrinkage. Where such a casting is used as parts such as passages or containers for gas, leakage might be caused through such parts due to communication of pores or shrinkage cavities within the casting. If defects within the casting, even if they are pores due to hydrogen gas or shrinkage, are present independently, leakage through such parts does not take place.
Pores, which are formed within an aluminum alloy casting obtained by a common casting method such as gravity casting or low pressure casting, are present in grain boundaries 5 of crystal grains 1 as shown in FIG. 5. In addition, as shown in FIG. 5, pores are not circular and when the grain size is large, pores 3a and 3b are easily communicated.
When an aluminum alloy casting having pores of the aforementioned shape is used for intake system parts for automobiles such as a manifold and a collector whose inside and outside pressures are different, intake air may leak outside, resulting in the possibility of deteriorating engine performance. Therefore, various measures, e.g. a gassing treatment for removing hydrogen gas from the molten aluminum alloy, have been taken to prevent pores due to hydrogen gas from occurring.
When casting is made from the degassed molten aluminum alloy, however, formation of non-circular pores is surely reduced, but shrinkage cavity occurs conversely in thick wall portions at an area where wall thickness varies largely. For example, when shrinkage cavity 10 occurs at boss 7 of a manifold or a collector as shown in FIG. 6, a tapping process to the boss 7 communicates its interior and outside, resulting in the leakage of intake air outside.
Therefore, conducting the gassing treatment to prevent the occurrence of pores due to hydrogen gas leads to another defect in the casting, and it is difficult to stably obtain an aluminum alloy casting having excellent airtightness.
To remedy such a drawback, Japanese Patent Publication No. 5-65573 discloses an aluminum alloy for casting which prevents the occurrence of shrinkage cavities due to Fe impurities and has excellent airtightness by adding 0.001 to 0.01 wt % of Ca to an Al-Si-Mg system or Al-Si-Mg-Cu system alloy.
But, even if the shrinkage cavities are reduced by adding Ca, the shrinkage cavities have a non-circular shape, possibly resulting in communicating pores. Therefore, using an aluminum alloy casting having such pores for intake system parts for automobiles still has problems.
Additionally, Japanese Patent Application Laid-open Print No. 5-98379 discloses an aluminum alloy which is used for automobile intake system parts such as a manifold. This aluminum alloy containing 4.0 to 10.0% of Si, 4.5% or below of Cu, 0.5% or below of Mg, and the rest of Al and inevitable impurities in weight ratio, has the aspect ratio of pores (length of pore/breadth of pore) reduced, has the area rate of pores (generation rate of pores) reduced, and has the correlation of the aspect ratio and the pore area rate fixed within a certain range, thereby preventing the leakage due to pores from occurring at thin wall portions near thick wall portions.
The above application controls the generation rate of pores and the aspect ratio indicating the generation form of pores but does not solely control pores themselves.
As a method to prevent shrinkage cavities from occurring in the thick wall portions of a casting, hydrogen is added to molten metal to disperse shrinkage cavities. Hydrogen dissolved in the molten aluminum has its solubility sharply reduced when the molten aluminum solidifies, so that hydrogen is gasified into bubbles as shown by the following formula to leave fine pores behind in an aluminum alloy casting. Thus, it is known that hydrogen is effective to relieve shrinking due to solidification and to disperse shrinkage cavities into fine pores.
Formula: 2H (dissolution).fwdarw.H.sub.2 (gas) PA1 Formula: 2Al+3H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +6H (dissolution)
To add hydrogen to the molten aluminum, a method which has been used heretofore is dipping potato or water-soaked cloth into the molten metal to utilizes the decomposition of water as shown by the following formula.
Particularly, potato, which contains an appropriate amount of water, is often used because workability is excellent.
But, this method generates oxide of aluminum as shown in the above formula, producing a large quantity of slag. Therefore, work for raking out the slag is required. And, because of the inclusion of the oxide into the molten metal, a casting obtained from this molten metal contains a large quantity of film oxides, deteriorating the strength. Additionally, the film oxides induce the generation of gas, so that pores have long shapes to easily communicate with one another and to degrade airtightness.
Japanese Patent Publication No. 51-44084 discloses a method for producing a porous aluminum alloy in which quick solidification of the molten aluminum is prevented, 1-25% of magnesium is added to be dissolved to control the size of crystal grains, and titanium hydride generating gas at a temperature less than a liquidus line and above a solidus line of the alloy is added and stirred in order for bubbling and the improvement of viscosity. According to this publication, an aluminum alloy casting having bubbling property is obtained. The porosity of bubbling is approximately 30 to 60%, and the size of pores is about 0.05 to 0.20 mm.
But, the above aluminum alloy has an excessively large porosity, lacks in airtightness, and has insufficient mechanical strength, so that it cannot be used for automobile intake system parts.